U.S. Pat. No. 3,087,807 teaches a method which permits the manufacture of a porous metal body of any desired shape. According to this method, a mixture of a metal powder and a propellant powder is cold-compacted at a compressive pressure of at least 80 MPa in a first step. Subsequent extrusion molding reshapes it at least 87.5%. This high degree of conversion is necessary for the friction of the particles with one another during the shaping process to destroy the oxide coatings and bond the metal particles together. The extruded rod thus produced can be foamed to form a porous metal body by heating it at least to the melting point of the metal. Foaming can be performed in various molds so that the finished porous metal body has the desired shape. The disadvantage is that this method is costly because of its two-step compacting process and the very high degree of conversion required, and is limited to semifinished products that can be made in extrusion molds. The method disclosed in this U.S. patent can only use propellants whose decomposition temperature is above the compacting temperature, since otherwise the gas would escape during the heating process. However, propellants whose decomposition temperatures are below the compacting temperature are suitable for many types of metals and are economical. During the foaming which follows the compacting process, a porous metal body is produced with open porosity, i.e., the pores are open or connected together. The extrusion process according to the method described in the U.S. patent is necessary because bonding of the metal particles takes place as a result of the high temperatures that occur during the extrusion process and the friction of the particles against one another, in other words, by welding the particles together. Since for the reasons given above the temperature required for bonding the particles together cannot be set at an arbitrary level, very high degrees of conversion must be used to produce a bonding of the metal particles with one another which is as satisfactory and gas-tight as possible.
In addition, several methods are known in which porous metal materials can be produced. One simple method for producing these materials is mixing substances that split off gases into metal melts. Under the influence of temperature, the propellant decomposes, releasing gas. This process results in the foaming of the metal melt. When the process is complete, a foamed metal material is left which has an irregular random shape. This material can be processed further by suitable methods to produce bodies of desired shape. It is important to keep in mind, however, that only separating methods can be used as methods for further processing, and consequently not just any metal body can be shaped from such a metal material. This is disadvantageous. Other methods for producing porous metal materials suffer from similar disadvantages, including, for example, impregnating an existing plastic foam with a slurry of metal powder and a carrier medium and then burning off or evaporating the plastic foam after drying. Apart from the above-mentioned disadvantages, this method is very costly.